1. Field of the Invention
The present invention relates to a new method of forming a fine pattern through use of a nitride-silicon-based film. Particularly, the present invention relates to a method of forming a fine pattern, which can minimize the effectiveness of a standing wave in a photoresist film during a photolithography step at the time of formation of a fine pattern and can improve the stability of a nitride-silicon-based film during a device fabrication step. The present invention can be utilized as, for example, a method of forming a fine pattern in the course of fabrication of a semiconductor device.
2. Description of the Background Art
A technique of diminishing the amount of exposing radiation reflected by a substrate (i.e., an anti-reflection technique) has been known as a peripheral technique required for satisfying the dimensional precision and resolution needed in fabrication of an ULSI device. In the event that exposing radiation is reflected by a substrate, a thin-film interference phenomenon arises within a photosensitive thin film, such as a photoresist film. As a result of occurrence of the thin-film interference phenomenon, there arise variation in exposure light in the thicknesswise direction of the resist film, which inconsistencies are called standing waves, thereby resulting in deteriorated resolution of a resist pattern.
Further, in the event that exposing radiation is reflected by a substrate, there arise variations in the dimension of a pattern associated with variations in the thickness of a resist, the variations being called multiple interference. The variations in turn deteriorate the dimensional precision of the resist pattern. The exposing radiation reflected by the substrate randomly travels in an oblique direction, because of irregularities in the substrate. As a result, regions which are originally intended to be shielded may be exposed, thereby hindering formation of a desired pattern (i.e., resulting in occurrence of a halation phenomenon). The problem becomes more noticeable in proportion to the intensity of the light reflected from the substrate. Consequently, if the reflected light is diminished, the problem is prevented. For this reason the number of attempts to diminish the light reflected from the substrate becomes higher than ever.
Anti-reflection methods can be roughly divided into two categories. One of those is a method in which a so-called absorptive film or a film having the characteristic of strongly absorbing exposing radiation is employed as an anti-reflection film. The other is a method which prevent reflection utilizing light interference.
The former anti-reflection method is typified by an ARC (Anti-Reflective Coating) method under which an absorptive organic film is applied to a substrate in advance before coating of resist. The almost portion of light that has passed through a resist film and travels toward a substrate is absorbed by the absorptive organic film before the light reaches the surface of the substrate. Therefore, the intensity of the light which has returned from the substrate is diminished. The ARC method is described in xe2x80x9cProceeding of SPIE, Vol. 1463, pp. 16 to 29, 1991, as well as in Japanese Patent Application Laid-Open No. 93448/1984.
An example anti-reflection method using light interference is a method of depositing an anti-reflection film, such as SiOXNY or SiNX, on a high-reflection substrate such as Al, W, Si, or WSi. According to this method, the thickness of the anti-reflection film is set such that the light reflected from a boundary surface between a photoresist film and an anti-reflection film is reverse in phase with the light reflected from a boundary surface between the anti-reflection film and the substrate. In this case, the reflected light rays cancel each reflected light which enters the photoresist film.
The latter anti-reflection method is described in Japanese Patent Application Laid-Open Nos. 6540/1984, 130481/1982, xe2x80x9cProceeding of SPIExe2x80x9d Vol. 2197, pp. 722 to 732, 1994, and xe2x80x9cTechnical Digests of International Electronic Device Meetingxe2x80x9d pp. 399 to 402, 1982.
In a case where a substrate having a large step is subjected to the ARC method, as shown in FIG. 1, the portion of an anti-reflection film 102 located above the step becomes thinner than the portion of the same surrounding the step. For this reason, the thickness of the anti-reflection film 102 must be set to a sufficient value in consideration of the diminished thickness portion of the anti-reflection film located above the step. However, in a case where a thick anti-reflection film is used for forming a fine pattern, the ratio of the thickness of the anti-reflection film to the width of a pattern; that is, an aspect ratio, becomes very large. In this case, processing of an anti-reflection film becomes very difficult, wherewith a failure, such as a tilt, is likely to arise in the thus-formed pattern.
The anti-reflection film, which is made of SiOXNY or SINX and is used for preventing reflection through utilization of light interference, can be deposited by means of the CVD method. Even in a case where a step arises in the substrate, a uniform thickness can be attained. Therefore, the anti-reflection method utilizing light interference provides an anti-reflection effect better than that achieved by the ARC method.
The surface of an anti-reflection film, which is made of, e.g., SiOXNY or SiNX and has conventionally been used for the anti-reflection method utilizing light interference, contains a large amount of basic nitrogen. In a case where the substrate is exposed while a positive chemically-amplified resist is applied to the anti-reflection film, acid contained in the resist bonds to the lone pair of electrons of each of nitrogen atoms contained in the surface of the anti-reflection film during the course of a baking step (PEB step) to which the resist is to be subjected after exposure. As a result, there arises a reduction in the acid content in the boundary surface between the resist and the substrate.
The photoresist has the property such that an area whose acid content is decreased is less soluble in a developer solution. Because of this property, in the event of a reduction arising in the acid content in the boundary surface between the photoresist and the substrate, rounded corners are likely to arise in the resist pattern. The rounded corners are not preferable, because control of a pattern width is deteriorated by the rounded corners arising in the resist pattern.
In order to solve the problem, there has been proposed a method of applying a photoresist to a substrate after a nitrogen-free substance (for example, an SiO film prepared through use of the plasma CVD technique) has been deposited on the surface of an anti-reflection film (Japanese Patent Application Laid-Open No. 189441/1998). However, the test conducted by the inventors has shown that occurrence of rounded corners cannot be prevented by means of depositing an SiO film on an anti-reflection film.
In a case where an anti-reflection film, which is made of SiOXNY or SiNY, is deposited on a substrate at a temperature of, for example, less than 400xc2x0 C., by means of the plasma CVD technique, the resultant anti-reflection film contains a large amount of hydrogen atoms. If the device fabrication process is carried out while the anti-reflection film containing a large amount of hydrogen atoms is left in the semiconductor device, hydrogen contained in the anti-reflection film desorbs from the film and spreads in interconnections, which are made of amorphous silicon or copper, or an interlayer film, such as a BPSG, as the wafer undergoes various baking steps to be carried out during the course of the fabrication process. Spreading of hydrogen involves degradation of interconnections or an interlayer film, which in turn deteriorates the reliability of a semiconductor device.
For instance, in the case of the step shown in FIG. 4, the step of depositing the anti-reflection film 7 is followed by a step of forming an interlayer film 12 (for example, a BPSG film) and a reflow step. These steps are usually performed at a temperature ranging from 700 to 800xc2x0C., and the composition of the anti-reflection film 7 is changed. In a case where a nitride-silicon-based film is deposited according to the conventional method, the hydrogen contained in the anti-reflection film 7 (the hydrogen is contained in combination with an Sixe2x80x94H bond or Nxe2x80x94H bond) desorbs from the film, thus changing the composition of the anti-reflection film. Consequently, a change may arise in the optical constant of the anti-reflection film, or in internal stress of the anti-reflection film, or exfoliation of the anti-reflection film may arise. The anti-reflection film which has undergone such a change fails to act as an anti-reflection film and adversely affects various features of a semiconductor device.
Removal of the anti-reflection film immediately after the fine pattern formation step is effective for preventing degradation in a semiconductor device, which would otherwise be caused by desorption of hydrogen. However, such a measure for removing an anti-reflection film directly involves an increase in the number of manufacturing steps.
A low-pressure CVD technique is also conceivable as a method of depositing an anti-reflection film made of SiOXNY or SiNX. The low-pressure CVD technique enables a reduction in the concentration of hydrogen atoms contained in an anti-reflection film. However, deposition of an anti-reflection film through use of the low-pressure CVD technique involves a necessity of heating a substrate to a temperature of about 800xc2x0 C.
Heat treatment at such a high temperature may cause thermal deformation of a substrate. Further, an anti-reflection film deposited through low-pressure CVD usually has high internal stress, wherewith the substrate is likely to cause deformation. A substrate for use in fabricating a semiconductor device is expected to enlarge further and further in the future, and hence prevention of thermal deformation and internal stress of a substrate continue grow in importance.
As mentioned previously, the method of preventing the influence of reflected light through use of an anti-reflection film, which is made of SiOXNY or SiNX, is superior to the ARC method. However, the method involves problems; that is, (1) a problem of rounded corners being likely to arise in a resist pattern, (2) a problem of the reliability of a semiconductor device being likely to decrease, and (3) a problem of a substrate being apt to cause deformation.
The present invention has been conceived to solve such a problem and is aimed at providing a method of forming a fine pattern which can prevent deformation of a substrate and a reduction in the reliability of a semiconductor device and which enables accurate formation of a fine resist pattern.
The above objects of the present invention are achieved by a fine pattern formation method under which a fine pattern is formed by means of applying a photoresist over a underlying substrate, exposing the photoresist to light of a single wavelength so as to form a fine resist pattern, and etching the underlying substrate while the resist pattern is used as a mask. In the method, a silicon-nitride-based film is formed on the underlying substrate directly or by way of another layer. A photoresist is formed on the silicon-nitride-based film directly or by way of another layer. The photoresist is exposed to light, to thereby transfer a mask pattern onto the photoresist. The silicon-nitride-based film is etched while the thus-transferred resist pattern is taken as a mask.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.